1. Field of the Invention
The present invention relates to image sensors for converting an optical image into electrical signals and particularly to a method and apparatus for optimizing exposure time in image acquisitions.
2. Description of the Related Art
An image sensor is used to convert an optical image focused on the sensor into electrical signals. The image sensor typically includes an array of photodetectors, where each photodetector produces a signal corresponding to the intensity of light impinging upon that photodetector when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image.
One very common type of image sensor is a charge coupled device (CCD). Integrated circuit chips containing a CCD image sensor have a relatively low yield and are expensive due to the specialized processing involved. The CCDs also consume a relatively large amount of power. Other well known disadvantages exist.
A much less expensive type of image sensor is formed as an integrated circuit using a complementary metal-oxide semiconductor (CMOS) process. In such a CMOS type image sensor, a photodiode or phototransistor (or other suitable device) is used as the light detecting element, where the conductivity of the element corresponds to the intensity of light impinging upon the element. The variable signal thus generated by the light detecting element is an analog signal whose magnitude is approximately proportional (within a certain range) to the amount of light impinging on the element.
It is known to form these light detecting elements in a two dimensional core array which is addressable by row and column. Once a row of elements has been addressed, the analog signals from each of the light detecting elements in the row are coupled to the respective columns in the array. An analog-to-digital (A/D) converter may then be used to convert the analog signals on the columns to digital signals so as to provide only digital signals at the output of the image sensor chip.
Present technologies only address the speed and the reduction in size of the image sensor array. U.S. Pat. No. 5,461,425 to Fowler et al. discloses the use of pixel level A/D converters to speed up the conversion of the optical image into digital data streams. U.S. Pat. No. 5,801,657 to Fowler et al. discloses a method for simultaneously performing bit serial analog to digital conversion which also speeds up the conversion of an optical image into digital data streams. However, none of the present technologies have addressed the issue of photodetectors reaching saturation in certain areas of the image sensor array before the rest of the photodetectors in the image sensor array do. This causes problems with over or under exposure of the image sensor array.
A need therefore exists for providing a method and an apparatus to optimize the exposure times of regions of pixels in an image sensor array during the exposure.
One of the objectives of the present invention is to provide a method of optimizing the exposure times of regions of pixels of an image sensor array during the exposure. This is in contrast with present technology that requires 2 exposures in order to optimally capture 1 image. The first exposure is to determine the optimal exposure parameters and the second exposure is to actually capture the image using the optimal settings.
In view of the foregoing objectives, a method of the present invention utilizes time interval sampling to optimize the exposure of an image acquisition system. The digital signals generated by pixels in an image sensor array are passed through a color separator in order to extract luminance values for each pixel. These luminance values are statistically analyzed in order to determine the saturation levels of each pixel. A pixel in this invention comprises of a photodetector and analog-to-digital conversion circuitry. Because digital signals, as opposed to analog signals, are generated by the pixels, the read out of the digital signals from the image sensor array can be performed very quickly. This enables the present method of multiple time interval sampling during one exposure period.
During an exposure period, the image sensor array is sampled at multiple time intervals, to determine the saturation levels of each pixel. Once a pixel is determined to have reached an optimal saturation ceiling, the subsequent digital image signals from this pixel are not recorded. This preserves the optimal digital image signal that is generated by the pixel at the time when the pixel has reached the saturation ceiling.
This process of selectively terminating the recording of digital image signals based on saturation levels of the pixels can be performed on individual pixels or can be performed on a region of pixels.